The present invention relates to internal-combustion engines of the type comprising:
at least one induction valve and at least one exhaust valve for each cylinder, each valve being provided with respective elastic means that bring back the valve into the closed position to control communication between the respective induction and exhaust ducts and the combustion chamber;
a camshaft for operating the induction and exhaust valves of the cylinders of the engine by means of respective tappets, each induction valve and each exhaust valve being actuated by a cam of said camshaft;
in which at least one of said tappets controls the respective induction or exhaust valve against the action of said elastic return means via the interposition of hydraulic means including a hydraulic chamber containing fluid under pressure;
said hydraulic chamber containing fluid under pressure being connectable, via a solenoid valve, to an outlet channel for decoupling the valve from the respective tappet and causing fast closing of the valve under the action of respective elastic return means;
said hydraulic means further comprising a piston associated to the stem of the valve and slidably mounted in a guide bushing, said piston being set facing a variable-volume chamber defined by the piston inside the guide bushing, said variable-volume chamber being in communication with the hydraulic chamber containing fluid under pressure by means of an end aperture of said guide bushing, said piston having an end appendage designed to be inserted into said end aperture during the final stretch of the closing stroke of the valve in order to restrict the communication port between said variable-volume chamber and said hydraulic chamber containing fluid under pressure, so as to slow down the stroke of the valve in the proximity of its closing.
An engine of the type referred to above is, for example, described and illustrated in the European patent applications Nos. EP-A-0 803 642 and EP-A-1 091 097 filed by the present applicant.
Studies and tests carried out by the present applicant have shown that some problems may arise during operation, particularly at engine starting. When the engine has not been running for a long time, in fact, the hydraulic circuit of the valve-control system is emptied of oil under pressure. When the engine is restarted, the pump for feeding oil under pressure again sends the oil to fill the circuit, at the same time causing the air present in the circuit to come out. The air-bleed ways are, however, relatively few, which means that there is a delayed response of the system.
The purpose of the present invention is to overcome the above-mentioned problem.
With a view to achieving this purpose, the subject of the invention is an engine having all the characteristics referred to at the beginning of the present description and moreover characterized in that it comprises at least one passage for bleeding the hydraulic circuit, which sets the pressure chamber associated to each tappet actuated by the camshaft in communication with the outside environment, this passage being intercepted and closed following upon a displacement of the tappet from its resting position.
In one first embodiment, the bleed passage is defined between a flattened portion of the cylindrical surface of the tappet and the cylindrical wall of a bushing within which the tappet is slidably mounted. This solution presents the advantage that the bleed passage can be easily pre-set in the most favourable position by angular orientation of the tappet. This is advantageous if the tappet has an axis that is inclined with respect to the vertical in so far as the passage must be set on the higher side of the tappet.
In a second embodiment, the passage is made in the body of the bushing within which the tappet is slidably mounted, and the latter has an end portion of reduced diameter that defines an annular chamber into which the aforesaid passage comes out when the tappet is in its resting position, whereas the said passage is intercepted by the main portion, i.e., the wider portion, of the tappet when the latter moves away from its resting position. Alternatively, the passage may be defined by a slit in the guide bushing.
Of course, various other conformations and arrangements of the bleed passage are possible.